The invention finds application in the field of lithographic projection apparatus that incorporates a radiation system for supplying a projection beam of radiation, a support structure for supporting a patterning device, which serves to pattern the projection beam according to a desired pattern, a substrate table for holding a substrate; and a projection system for projecting the patterned beam onto a target portion of the substrate.
The term “patterning device” as employed here should be broadly interpreted as referring to devices that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning devices include:                A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmission mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.        A programmable mirror array. One example of such a device is a matrix-addressable surface having a visco-elastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as non-diffracted light. Using an appropriate filter, the said non-diffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localised electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic circuits. In both of the situations described here above, the patterning device can include one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, and from WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.        
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For simplicity, parts of the rest of this specification are directed specifically to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning device as set forth above.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper or step-and-repeat apparatus. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information on such lithographic devices is disclosed in U.S. Pat. No. 6,046,792, the contents of which are incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This combination of processing steps is used as a basis for patterning of a single layer of the device which is for example an integrated circuit (IC). Such a patterned layer may then undergo various processes, such as etching, ion-implantation (doping), metallisation, oxidation, chemical-mechanical polishing, etc., all of which are intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be produced on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, so that the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processing can be obtained, for example, from “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For simplicity, the projection system may hereinafter be referred to as the “lens”. However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”.
Furthermore, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus is described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, the contents of both of which are incorporated herein by reference.
Although specific reference may be made in this specification to the use of the apparatus according to the invention in the manufacture of integrated circuits, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The person skilled in the art will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” in this text should be considered as being replaced by the more general terms “substrate” and “target portion”, respectively. Generally, throughout the specification, any use of the term “mask” should be considered as encompassing within its scope the use of the term “reticle”
In the present document, the terms “radiation” and “projection beam” are used to encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range 5-20 nm).
The phenomenon of lens heating can occur in the projection system of a lithographic projection apparatus. The projection lens becomes slightly heated by the projection beam radiation during exposures. As a result of this heating, refractive index changes occur, and a certain expansion of lens elements occurs, causing subtle changes in the geometric form of those elements, with an attendant change in their optical properties. This can result in the occurrence of new lens aberrations, or a change in existing aberrations. Because the occurrence of these aberration changes depends on such matters as the particular lens geometry, lens material, projection wavelength, light source power, target portion, wafer-reflectivity, size, and so on, the accuracy with which the effects of such lens heating can be predicted can be limited, especially in the absence of any measurement and compensating mechanism.
Lens heating has always occurred to some extent in lithographic projection apparatus. However, with the trend to integrating an ever-increasing number of electronic components, and thus smaller features, in an IC, and to increase the manufacturing throughput, shorter wavelength radiation, such as EUV radiation has been used, as well as high-power radiation sources, such as 3-6 kW Mercury-arc lamps and excimer lasers with a power of 10 to 20 W, which together with the reduction in feature size have made lens heating a more serious problem. The problem is generally worse in scanners than in steppers because, in a stepper, substantially the whole (circular) cross-section of each lens element is irradiated, whereas, in a scanner, generally only a slit-shaped portion of the lens elements is irradiated; consequently, the effect in a scanner is far more differential than in a stepper, even if the lens aberrations in the scan direction are averaged out in the scanner, thereby resulting in the occurrence of new lens aberrations.
The change in the optical properties of the elements of the projection system due to such lens heating naturally affects the image that is projected, principally by causing a change in the image parameters, of which magnification is particularly important for the XY-plane, and focus is particularly important for the Z-plane. However this lens heating effect can be calibrated and compensated for very well, e.g. by adjusting the positions of the lens elements to effect a compensating change in magnification or other image parameters of the projection system, for example as disclosed in EP 1164436A or U.S. Pat. No. 6,563,564, the contents of both of which are incorporated herein by reference. The lens heating effects depend on the lens properties, which are calibrated when the apparatus is constructed and may be recalibrated periodically thereafter, and various parameters of the exposures carried out, such as mask transmission, dose, illumination settings, field size and substrate reflectivity.
When performing imaging in a lithographic projection apparatus, despite the great care with which the projection system is designed and the very high accuracy with which the system is manufactured and controlled during operation, the image can still be subject to aberrations, which can cause offsets in the image parameters, such as, for example, distortion (i.e. a non-uniform image displacement in the target portion at the image plane: the XY-plane), lateral image shift (i.e. a uniform image displacement in the target portion at the image plane), image rotation, and focal plane deformation (i.e. a non-uniform image displacement in the Z-direction, for instance field curvature). It should be noted that, in general, image parameter offsets are not necessarily uniform, and can vary as a function of position in the image field. Distortion and focal plane deformation can lead to overlay and focus errors, for example overlay errors between different mask structures, and line-width errors. As the size of features to be imaged decreases, these errors can become intolerable.
Consequently, it is desirable to provide compensation (such as adjustment of the projection system and/or substrate) to correct for, or at least attempt to minimize, these errors. This presents the problems of first measuring the errors and then calculating appropriate compensation. Previously, alignment systems were used to measure the displacements in the image field of alignment marks. However, alignment marks typically consist of relatively large features (of the order of a few microns), causing them to be very sensitive to specific aberrations of the projection system. The alignment marks are unrepresentative of the actual features being imaged, and, because the imaging errors depend inter alia on feature size, the displacements measured and compensations calculated do not necessarily optimize the image for the desired features.
Another problem occurs when, for instance, because of residual manufacturing errors, the projection system features asymmetric variations of aberrations over the field. These variations may be such that at the edge of the field the aberrations become intolerable.
A further problem occurs when using phase-shift masks (PSM's). Conventionally, the phase shift in such masks has to be precisely 180 degrees. The control of the phase is critical; deviation from 180 degrees is detrimental. PSM's, which are expensive to make, must be carefully inspected, and any masks with substantial deviation in phase shift from 180 degrees will generally be rejected. This leads to increased mask prices.
A further problem occurs with the increasing requirements imposed on the control of critical dimension (CD). The critical dimension is the smallest width of a line, or the smallest space between two lines, permitted in the fabrication of a device. In particular the control of the uniformity of CD, the so-called CD uniformity, is of importance. In lithography, efforts to achieve better line width control and CD uniformity have recently led to the definition and study of particular error types occurring in features obtained during exposure and processing. For instance, such image error types may include an asymmetric distribution of CD over a target portion, an asymmetry of CD with respect to defocus (which results in a tilt of Bossung curves), asymmetries of CD within a feature comprising a plurality of bars (commonly referred to as Left-Right asymmetry), asymmetries of CD within a feature comprising either two or five bars (commonly known as L1-L2 and L1-L5, respectively), differences of CD between patterns that are substantially directed along two mutually orthogonal directions (for instance the so-called H-V lithographic error), and for instance a variation of CD within a feature, along a bar, commonly known as C-D. As in the case of the aberrations mentioned above, these errors are generally non-uniform over the field. For simplicity we will hereafter refer to any of these error types, including errors such as, for example, distortion, lateral image shift, image rotation, and focal plane deformation, as lithographic errors, i.e. feature-deficiencies of relevance for the lithographer.
Lithographic errors are caused by specific properties of the lithographic projection apparatus. For instance, aberrations of the projection system, or imperfections of the patterning device and imperfections of patterns generated by the patterning device, or imperfections of the projection beam may cause such lithographic errors. However, also nominal properties (i.e. properties as designed) of the lithographic projection apparatus may cause unwanted lithographic errors. For instance, residual lens aberrations which are part of the nominal design may cause lithographic errors. For reference hereafter, we will refer to any such properties that may cause lithographic errors as “properties”.
As mentioned above, the image of a pattern can be subject to aberrations of the projection system. A resulting variation of CD (for example, within a target portion) can be measured and subsequently mapped to an effective aberration condition of the projection system which could produce the measured CD variation. A compensation can then be applied to the lithographic projection system so as to improve CD uniformity. Such a CD control method is described in U.S. Pat. No. 6,115,108, incorporated herein by reference, and comprises imaging of a plurality of test patterns at each field point of a plurality of field points, subsequent processing of the exposed substrate, and subsequent CD measurement for each of the imaged and processed test patterns. Consequently, the method is time consuming and not suitable for in-situ CD control. With increasing demands on throughput (i.e. the number of substrates that can be processed in a unit of time) as well as CD uniformity, there is a need for the control, compensation and balancing of lithographic errors to be improved, and hence there is a need for further appropriate control of the properties.
U.S. Pat. No. 6,563,564 (P-0190) discloses a lens heating model by which projection system aberrations due to the lens heating effect can be corrected for by way of image parameter offset control signals that serve to adjust the image parameters of the projection system to compensate for the calculated change in the aberration effect due to such lens heating. In this case the change in the aberration effect with time is determined on the basis of a stored set of predetermined parameters corresponding to the selected aberration effect, these parameters preferably being obtained by a calibration step. The image parameter offsets may comprise focus drift, field curvature, magnification drift, third-order distortion, and combinations thereof. However, the required ideal compensation will depend on the particular application (the particular pattern, illumination mode, etc.), and the number of parameters that can be adjusted is generally not high enough to cancel out every aberration completely, so that the determination of the compensation to apply in a particular case will always be a compromise, the particular compromise to be chosen depending on the required application. Because the conventional lens heating model does not take into account the particular application, it follows that the calculated compensation will not be optimal for every particular application.
EP 1251402A1 (P-0244) discloses an arrangement for compensating for projection system aberrations on the basis of the relationship between the properties of the substrate, the layer of radiation sensitive material on the substrate, the projection beam, the patterning device and the projection system, and the lithographic errors causing anomalies in the projected image. A control system determines a merit function which weighs and sums the lithographic errors, and calculates a compensation to apply to at least one of the substrate, the projection beam, the patterning device and the projection system to optimize the merit function. Although the use of such a merit function enables compensation to be applied in such a manner as to reach a reasonable compromise in terms of optimization of the image, it is found that, since such optimization is intended to provide the best compromise in terms of imaging quality over the whole of the image, the image quality in parts of the image or in particular applications may be relatively low.
A control system may be provided for compensating for the effect of changes in a property of lithographic projection apparatus with time, such as the change in magnification of the projection system due to lens heating, in which a control signal is generated according to a predicted change in the property with time according to a defined model, a comparator compares a value based on the predicted change to a threshold, and generates a trigger signal when the value is greater than the threshold value, and the alignment system performs an alignment in response to the trigger signal. Such an arrangement triggers a so-called “realignment” when the predictive correction becomes larger than the desired maximum. This system therefore predicts the heating effects that will occur in performing a series of exposures and applies appropriate corrections in advance of the exposures being made when the corresponding threshold value is exceeded. This technique ensures that realignments occur only when errors are out of certain ranges, and avoids unnecessary realignments, thus avoiding loss of throughput in the exposure process. In certain applications errors in the predictive correction may result in unnecessary additional alignment steps and loss of throughput, since the optimal time for realignment is not calculated on the basis of the particular application. This could mean in practice that the imaging performance is worse than expected in a particular series of exposures due to the realignment being triggered too late for the particular application; or that the throughput is less than expected due to the realignment being effected sooner than required in the series of exposures.
Furthermore, since the model is in many situations non-linear with respect to parameters such as illumination setting and energy dose, it is necessary for the model to be calibrated for the particular application that is to be performed, that is at a particular working point (illumination setting, energy dose, etc.). The model can then be used by interpolation for modeling the performance at non-calibrated working points. However calibration of the model for use at a selected working point, that is at a fixed energy dose and illumination level, can be very time-consuming due the offline nature of the calibration process, and this can mean that the availability of the lithographic projection apparatus is reduced for at least eight hours. Additionally the calibrated model may still be inaccurate because the lens heating is dependent on the combination of product structure and illumination mode, and the interpolation to other working points may be poor.